1. Field of the Invention
This invention relates generally to a flexible circuit board and a production method therefor and, more specifically, to a flexible circuit board having external connection terminal portions formed like lands or bumps to be electrically connected with various electric or electronic parts or a circuit wiring portion on both front and rear surfaces of a circuit wiring pattern and a production method therefor.
2. Description of the Related Art
This type of flexible circuit board is now widely used in various fields as wiring means suitable for use in electronic devices and the like. Particularly, as means for producing a flexible circuit board having fine external connection terminal portions, there is known a technique shown in FIG. 2.
That is, as shown in FIG. 2a, a desired circuit wiring pattern 23 is first formed on one side of an appropriate flexible insulating base material 20. And a metal mask 21 required for the processing of removing part of the insulating base material 20 which will be described in the subsequent step is formed on the other side of the insulating base material 20. A hole 22 is suitably formed in the metal mask 21 by photoetching means at a position corresponding to a predetermined portion of the circuit wiring pattern 23.
An adhesive 24 is used to adhere an appropriate insulating film 25 to the exposed side of the circuit wiring pattern 23 to form a surface protective layer. During formation of this surface protective layer, as shown in FIG. 2a, a predetermined portion of the circuit wiring pattern 23 is exposed to form a conductive hole 26. This surface protective layer may be formed by photolithography means which uses an appropriate photosensitive resin material, in addition to the above technique which employs the above adhesive 24 and the insulating film 25. Thereby, a desired conductive hole 26 can be formed finely with high accuracy.
Then, as shown in FIG. 2b, an excimer laser beam is irradiated from the side of the metal mask 21 to abrasion remove a portion of the flexible insulating base material 20 located at the position of the hole 22 which has been described above to form a conductive hole 27 which exposes part of the surface of the circuit wiring pattern 23.
Thereafter, as shown in FIG. 2c, while the surface of the circuit wiring pattern 23 exposed on the bottom of the conductive hole 27 is suitably protected, the unnecessary metal mask 21 is removed by etching means and then, as shown in FIG. 2d, a conductive metal such as copper is filled into the conductive holes 26 and 27 by plating means. This forms bumps 28 and 29 one ends of which are electrically connected with predetermined portions of the circuit wiring pattern 23 and the other ends of which project outward suitably can be formed.
However, the above production method for a flexible circuit board employs a double-sided copper laminated sheet as a material for forming a desired circuit wiring pattern 23 and metal mask 21 on the respective sides of the flexible insulating base material 20 separately. This double-sided copper laminated sheet is generally expensive.
As means for forming the conductive hole 27 at a predetermined position of the circuit wiring pattern 23, an excimer laser machine must be used. However, this machine is extremely expensive and it is therefore very difficult to provide this type of flexible circuit board having fine external connection terminal portions at a low cost in such circumstances.